This invention relates to readers for magnetically coded cards such as are used in access control systems. A typical card is made of plastic with magnetic material embedded therein. In one form, individual magnets are positioned in the card. In the presently preferred form, a piece of magnetic material is laminated in the card and then spots are selectively magnetized along the magnetic material to provide a specific code. A typical card reader has a guide for slidingly receiving the card. The magnetic fields produced by the card may be used to mechanically move locking pins in the reader providing direct unlocking. Alternatively in more complex systems, the magnetic fields or the absence of magnetic fields at specific locations are sensed to provide a set of electrical signals as an output corresponding to the code of the card inserted in the reader.
In one type of card reader, devices are provided for sensing the presence or absence and polarity of the magnetic field produced by the card at every possible location of a magnetic spot on the card, with the sensing being done simultaneously so that a quantity of N bits require N sensors and at least N+1 data lines for transmission of the sensed information. Typical cards may utilize a 10 bit code with eleven output lines in parallel being required. A sensor of this type using magnetic reed switches as the sensors as shown in U.S. Pat. No. 3,581,030. Another reader, utilizing sensing coils, is shown In U.S. Pat. Nos. 3,717,749 and 3,780,268. Hall effect transducers have also been used for magnetic sensing. The readers using sense coils and Hall effect transducers generally require electronic amplification for the sense signals. The readers identified above are parallel type devices, requiring N+1 wires for data transmission from N sensors.
Serial type readers have been utilized with a single magnetic sensor with the card moving relative to the sensor. However, reliable operation demands a constant relative motion between the sensor and the card because of the sensor's dependence on the rate of change of magnetic flux (d.phi./dt). Hall effect devices have been considered for serial readers because they respond to magnetic field strength rather than rate of change of field but are not satisfactory because of their relatively large size, high cost, general difficulty in usage, and requirement of four wires for operation.
The magnetodiode is a semiconductor device, the resistance of which changes as a function of magnetic field strength applied to the device. The magnetodiode has been proposed as a sensor for magnetically coded cards; however, because of its relatively poor temperature characteristic, the diode has not been satisfactory. It is an object of the present invention to provide a new and improved reader for magnetically coded cards, incorporating a magnetodiode as the sensor. A particular object is to provide such a reader with output circuitry eliminating the adverse effects normally resulting from changes in ambient temperature.
It is an additional object of the invention to provide such a reader which can provide a serial output requiring only two wires, i.e., a signal wire and a ground return. A further object is to provide such a reader which is not dependent on the velocity of relative motion of the card and sensor and which does not require a constant velocity for operation. An additional object is to provide such a reader with an output which is self-clocking.